Single field shapes quantum
bits

By
Eric Smalley,
Technology Research NewsQuantum
computers, which tap the properties of particles like atoms, photons and
electrons to carry out computations, could potentially use a variety of
schemes: individual photons controlled by optical networks, clouds of atoms
linked by laser beams, and electrons trapped in quantum dots embedded in
silicon chips.

Due to the strange nature of quantum particles, quantum computers
are theoretically much faster than ordinary computers at solving certain
large problems, like cracking secret codes.

Chip-based quantum computers would have a distinct advantage: the
potential to leverage the extensive experience and manufacturing infrastructure
of the semiconductor industry. Controlling individual electrons, however,
is extremely challenging.

Researchers have recently realized that it may be possible to control
the electrons in a quantum computer using a single magnetic field rather
than having to produce extremely small, precisely focused magnetic fields
for each electron.

Researchers from the University of Toronto and the University of
Wisconsin at Madison have advanced this idea with a scheme that allows individual
electrons to serve as the quantum bits that store and process computer information.
The scheme is an improvement over existing global magnetic field schemes,
which require each qubit to consist of two or more electrons.

Electrons have two magnetic orientations, spin up and spin down,
which can represent the 1s and 0s of computing. The logic of quantum computing
is based on one-qubit gates and two-qubit gates. One-qubit gates flip individual
spins, changing a 1 to a 0 and vice versa. Two-qubit gates cause two spins
to become linked, or entangled.

The researchers' scheme relies on the interactions of pairs of electrons
to create both types of gates. Tiny electrodes positioned near quantum dots
-- bits of semiconductor material that can trap single electrons -- can
draw neighboring electrons near enough that they exchange energy. If the
electrons interact long enough, they swap spin orientations. The challenge
is finding a way to use the interaction to flip the spin of one electron
without flipping the spin of the other.

The scheme does so by taking a pair of electrons through eleven
incremental steps using the electron interaction and the global magnetic
field. "We first turn on the exchange interactions... through small electrodes
to generate a swap gate, then turn on the global magnetic field," said Lian-Ao
Wu, a research associate at the University of Toronto.

The eleven steps -- four electron interactions and seven pulses
of the magnetic field -- alter the spins. Because the magnetic field diminishes
in strength over distance each electron is exposed to a different strength.
By tuning the field, the researchers can make the process cancel out the
changes to one spin while flipping the other, according to Wu.

The researchers' scheme could be implemented using a pair of square,
100-nanometer-diameter aluminum nanowires separated by a thin insulating
layer. A row of quantum dots in a zigzag pattern would be positioned parallel
to the wires, with half of the dots 200 nanometers from the wires and the
other half 300 nanometers away. A nanometer is one millionth of a millimeter,
or the span of 10 hydrogen atoms.

The ability to build such a quantum computer depends on developments
in nanotechnology, said Wu. "It is still hard to design a complete control
scheme of the exchange interactions," he said. "Once such obstacles are
overcome, our scheme should offer significant simplifications and flexibility."

The on-chip conducting wires called for in the researchers' scheme
have been used in physics experiments involving controlling beams of atoms
and Bose-Einstein condensates, which are small clusters of atoms induced
to behave as one quantum entity, according to Wu.

The researchers are working on reducing the number of steps required
for their quantum logic circuit, combining their scheme with quantum error
correction techniques, and reducing the engineering challenge of implementing
the design, said Wu. The scheme would require making the aluminum wires
with a precision of a single layer of atoms, but optimizing the scheme should
make it possible to loosen the requirements to several atomic layers, which
is technologically feasible, according to Wu.

"The main challenge is [achieving a] high degree of control of the
exchange interactions," he said.

The technique could be used practically in 10 to 20 years, said
Wu.

Wu's research colleague was Daniel A. Lidar at the University of
Toronto and Mark Friesen at the University of Wisconsin at Madison. The
work appeared in the July 15, 2004 issue of Physical Review Letters.
The research was funded by the Defense Advanced Research Projects Agency
(DARPA), the National Science Foundation (NSF), and the Army Research Office/Advanced
Research and Development Activity (ARO/ARDA).